1. Field of the Invention
The present invention relates generally to invisibly marking articles and materials for a dual mode readout, using luminescent and optically stimulable glasses. More particularly, the invention relates to using metal ion dopants in a glassy matrix in inks, dyes, and the like, for marking articles and materials, and for detecting these materials by fluorescence and optically stimulated readout of trapped states. Particular aspects of the invention include luminescent doped glasses, including copper-, europium-, and cerium-doped glasses such as Vycor.TM. (a registered trademark of Corning Corporation, Corning, N.Y.) and fused quartz.
2. Description of the Related Art
Labels that are not apparent to the naked eye are desired for many manufactured articles and materials. It is also desired to make such labels as flexible in applicability and as robust as possible. For instance, it is desirable to make labels that can be applied with dyes, inks, stains, shellacs, varnishes, glazes, polymer coatings such as polyurethane coatings, and other coatings or materials for application to an article. It is also desirable to make such labels robust so that, for instance, they can survive exposure to moderately high temperatures and exposure to common solvents. It is also desired to make labels that are non-toxic.
It is also desired to make labels that have some type of redundancy built into them. That is, it is desired to make labels that can be read in more than one way, as a double check on the authenticity of a labeled article.
Since labels generally require some type of coding (bar coding, alphanumeric coding, etc.), it is desired to have such coding on a label that is likewise not visible to the naked eye. In practice today, this is often done by putting a "hidden" number such as a serial number in some out of the way place in an article, such as inside a case or housing. The difficulty is that such placement is almost inherently not readily accessible, and so it is difficult to get to when it is desired to read the label. It would be preferred to have coding that is hidden in plain sight. Coding that requires stimulation for readability would satisfy this need.
It is often desirable to have a way of monitoring some time relating to labeled articles, such as shipping time or shelf time. A label that included some type of timing capability would satisfy this need. Optimally, such a timing system would have a broad range, being able to time over scales of days, weeks, months, or years, as needed.
Materials used for detecting infrared light by stimulating upconverted visible luminescence have been intensively studied for decades. Early work during the 1940's determined that the most efficient materials were polycrystalline alkaline earth sulfides such as SrS or CaS doped with trivalent rare earth activator and coactivator ions such as Sm paired with Ce or Eu. Presently, the most widely used optically stimulable luminescent (OSL) phosphors have evolved very little from the early formulations. These phosphors are typically micron-sized powders or polycrystalline films that absorb ionizing radiation (.gamma.-ray, x-ray, .beta.-particle) or UV radiation and store a portion of the absorbed energy in the form of trapped charges. The trapped charges are stable until the phosphor is illuminated with a light source capable of stimulating luminescent electron-hole recombination.
Traditional OSL phosphors are highly scattering to visible light, due to the micron-sized features of the powders or films. The OSL is severely attenuated due to the scattering, thereby not only reducing the brightness of the emission, but also placing a practical limitation on the thickness and functionality of the phosphor for many applications. However, for applications where the phosphor is expected to be very thin, this scattering will be less critical.
The inventors have recently disclosed patents and published papers that describe several types of optically transparent, thermoluminescent, and optically stimulable glasses that operate through a charge trapping mechanism. For the thermoluminescent glasses, luminescent recombination of electron-hole pairs is induced by applying enough heat (by external heating or laser heating) to overcome the thermal barrier to recombination. In the case of an optically stimulable glass, recombination is induced by exposure to a light source with a wavelength that overlaps the optical stimulation band in the OSL material. Optical stimulation has a number of advantages over thermal stimulation, such as little or no bulk heating of the glass and a very rapid readout.